Components in a fluid stream can be converted or removed by contacting the fluid in axial flow with a fixed bed of granular material containing specific substances which physically or chemically interact with the components in the fluid stream. Examples include adsorption processes to separate or purify gases or liquids, catalytic chemical reaction processes, and the removal of contaminants from liquids by ion exchange. In these applications, the granular material can be subjected to considerable hydraulic forces by fluid flowing in an upward direction, which can result in undesirable movement of the granular material.
In a pressure swing adsorption process, for example, an adsorbent bed can be subjected to high gas flow velocities at certain times during each process cycle. The pressure swing adsorption process cycle includes the basic steps of adsorption, depressurization, purge, and repressurization, and also may include pressure equalization and provide purge steps in which gas is transferred from a bed at decreasing pressure to another bed which is at constant or increasing pressure. In certain of these steps, gas flows through the bed in an upward direction, and if the gas flow rate is sufficiently high, bed lifting or adsorbent fluidization can occur.
Bed lifting can occur when a high flow rate of gas is introduced into the bottom of an axial flow bed while little or no gas flows from the top of the bed, for example, during feed repressurization. The forces generated in the bottom portion of the bed as a result of pressure drop can exceed the weight of the bed, thus causing the entire bed to lift.
Fluidization may occur in the upper portion of the bed due to high gas velocity in that part of the bed. This situation can occur during pressure equalization or provide purge steps, for example, in which gas flows from the top of the bed at a high rate while no gas flows into the bottom of the bed. If the pressure drop across the top layer of the bed exceeds the weight of the adsorbent particles, fluidization will occur.
The phenomena of bed lifting and fluidization are undesirable and can adversely affect the integrity of beds of granular material. Bed mixing, channeling, dusting, and material carryover can occur which could require shutdown of the process for corrective maintenance and, in certain cases, could require discharging and refilling of granular material in the vessel. While it is possible to design fluid flow control systems so that upward fluid pressure drop will not cause excessive lifting forces on the granular material, this approach involves some risk that bed lifting and fluidization may still occur due to instrument failure. It is desirable to eliminate such risk by designing the vessels such that bed lifting and fluidization cannot occur. The invention described below and defined in the claims which follow offers specific bed designs which eliminate bed lifting and fluidization in processes which treat fluid streams.
The invention relates to a system for restraining the upward motion of granular material in a vessel containing a bed of the granular material through which a fluid flows in an upward direction. The system comprises:
(a) a flexible porous basket within the vessel in contact with the top of the bed of granular material and in contact with the inner walls of the vessel above the bed of granular material, wherein the porous basket has openings which are smaller than the smallest particles of the granular material such that granular material does not pass through the openings; and
(b) a layer of solid bodies located within the flexible porous basket wherein the solid bodies press the flexible porous basket against the top of the bed of granular material and against the inner walls of the cylindrical vessel above the bed of granular material, wherein the solid bodies in the layer of solid bodies have an average diameter greater than the average particle diameter of the granular material and the material forming the solid bodies has a density greater than the bulk density of the granular material.
The granular material may be adsorbent material and the fluid may be a gas. The solid bodies in the layer of solid bodies may be formed of material selected from the group consisting of mineral, ceramic, and metal. The density of the material forming the solid bodies in the layer of solid bodies typically nay be between about 1.5 and about 8 times the bulk density of the granular material. The solid bodies in the layer of solid bodies may comprise ceramic balls.
The average diameter of the solid bodies in the layer of solid bodies may be between about 1.5 and about 3 times the average particle diameter of the granular material. The depth of the layer of solid bodies typically may be between about 3 and about 6 inches.
A plurality of additional solid bodies may be located on top of the layer of solid bodies. The average diameter of these additional solid bodies may be between about 10 and about 50 mm. At least some of these additional solid bodies may be located within the flexible porous basket. The additional solid bodies may be formed of material selected from the group consisting of mineral, ceramic, and metal.
The solid bodies in the layer of solid bodies may be formed of material selected from the group consisting of mineral, ceramic, and metal. These solid bodies in the layer of solid bodies and the additional solid bodies may comprise ceramic balls.
The cylindrical vessel has an upper head and a lower head. At least a portion of the additional solid bodies may be in contact with, and restrained from upward movement by, the upper head of the cylindrical vessel. At least some of the additional solid bodies may be in contact with, and restrained from upward movement by, a perforated strainer assembly which is in contact with the upper head of the cylindrical vessel.
Optionally, at least a portion of the additional solid bodies may be restrained from upward movement by the mechanical application of downward force on the solid bodies in the second layer.
The invention also relates to an adsorber assembly which comprises:
(a) a cylindrical vessel having an upper head and a lower head;
(b) fluid inlet and outlet piping means connected to the lower head of vessel and fluid inlet and outlet piping means connected upper head for withdrawing fluid from the vessel;
(c) a bed of granular adsorbent material which partially fills the vessel;
(d) a flexible porous basket within the cylindrical vessel in contact with the top of the bed of granular material and with the inner walls of the vessel above the top of the bed of granular material, wherein the porous basket has openings which are smaller than the smallest particles of the granular material such that granular material does not pass through the openings; and
(e) a layer of solid bodies located within the flexible porous basket which press the flexible porous basket against the top of the bed of granular material and against the inner walls of the cylindrical vessel above the bed of granular material, wherein the solid bodies in the layer of solid bodies have an average diameter greater than the average particle diameter of the granular material and the material forming the solid bodies has a density greater than the bulk density of the granular material.
In the adsorber assembly, the depth of the layer of solid bodies typically may be between about 3 and about 6 inches. A plurality of additional solid bodies may be located on top of the layer of solid bodies. The average diameter of these additional solid bodies may be between about 10 and about 50 mm.
The adsorber assembly may further comprise a perforated strainer assembly located between the additional solid bodies and the upper head of the cylindrical vessel, wherein at least some of the solid bodies may be in contact with the upper head and the perforated strainer assembly.
The invention also relates to a method for restraining the upward motion of granular material in a vessel having an upper head and a lower head, which vessel contains a bed of granular material through which a fluid flows in an upward direction, wherein the method comprises:
(a) providing a flexible porous basket which has openings with sizes smaller than the smallest particles in the granular material such that granular material does not pass through the openings in the porous basket;
(b) installing the flexible porous basket into the cylindrical vessel in contact with the top of the bed of granular material and against the inner walls of the cylindrical vessel above the top of the bed of granular material; and
(c) placing a layer of solid bodies within the flexible porous basket which presses the flexible porous basket against the top of the bed of granular material and against the walls of the cylindrical vessel above the top of the bed of granular material, wherein the solid bodies in the layer of solid bodies have an average diameter greater than the average particle diameter of the granular material. The method may further comprise the additional features of:
(e) placing a plurality of additional solid bodies on top of the layer of solid bodies, wherein the average diameter of the additional solid bodies is between about 10 and about 50 mm; and
(f) placing a perforated strainer assembly between the additional solid bodies and the upper head of the cylindrical vessel, wherein at least some of the solid bodies are in contact with the upper head and the perforated strainer assembly.